Virtual machines can be provided in a computer to enhance flexibility and performance. A virtual machine typically refers to some arrangement of components (software and/or hardware) for virtualizing or emulating an actual computer, where the virtual machine can include an operating system and software applications. Virtual machines can allow different operating systems to be deployed on the same computer, such that applications written for different operating systems can be executed in different virtual machines (that contain corresponding operating systems) in the same computer. Moreover, the operating system in a virtual machine can be different from the host operating system that may be running on the computer on which the virtual machine is deployed.
In addition, a greater level of isolation is provided between or among applications running in different virtual machines than is provided by running applications in a single virtual or physical machine. In some cases, virtual machines also allow multiple applications to more efficiently share common resources (processing resources, input/output or I/O resources, and storage resources) of the computer. Such sharing of hardware resources is performed by virtualizing the hardware resources with a virtual machine control entity, such as a hypervisor.
For enhanced performance, virtual machines can be provided on multiple physical machines (e.g., computers) that are interconnected by a network. In some implementations, an automated placement controller has been provided to determine computers that the virtual machine is to be deployed on. Based on predetermined criteria, the placement controller is able to migrate at least some of the virtual machines across different computers.
The predefined criteria may include predefined migration constraints that limit migration of certain virtual machines to particular physical machines. In some instances, such migration constraints may be overly conservative in order to decrease the likelihood that migration of a virtual machine may result in an undesirable situation. For instance, a constraint may be created that limits migration of a particular virtual machine to only those physical machines having a processor that is compatible with the processor on which the virtual machine initially was placed. However, as it oftentimes is likely that the applications executing on that particular virtual machine can just as readily run on a physical machine having an incompatible processor (i.e., a processor supporting a different instruction set), such a migration constraint may, in many cases, needlessly limit the flexibility of the automated placement controller.